An ultrasonic scanner may be comprised of various types of materials. Normally, the ultrasonic energy used in such a scanner is required to pass through most of these materials. The properties of the various materials through which an ultrasonic wave passes or strikes may have differing properties with regard to dispersion, diffraction, absorption and reflection such that the materials will disperse, diffract, absorb, and reflect the ultrasonic energy in different ways, and these differences may be dependent upon the wavelength of the ultrasonic energy. Use of a single ultrasonic frequency to image a particular object may result in limited information and detail about the object being imaged.
During manufacture of an ultrasonic sensor array, tolerances may build up within the ultrasonic sensor stack that affect the signal path and may create a situation where the data collected does make use of the optimum available signal and response of the system. Furthermore, the data quality may be frequency dependent and the structural makeup of the target may present frequency dependencies.
Normal variations attributable to manufacturing ultrasonic scanning systems may result in one ultrasonic scanning system performing in a manner that is noticeably different from another, even though both scanning systems are manufactured within desired tolerances and according to the same procedures. A result of these differences may mean that one scanner collects information at an optimum frequency, while another scanner does not.
The basic methodology that has been applied in the prior art has been to perform a scan at a single specific frequency which maximizes the signal output as captured by a thin-film transistor (TFT) array positioned within the sensor stack. The single frequency may be primarily determined by the thickness and the material properties of the sensor stack and used to differentiate the fingerprint ridge and valley regions of a finger being imaged. In a manufacturing setting (without fingerprint references), the frequency determination may be made by choosing the frequency at which the sensor array output is maximized between two cases, one with the ultrasonic transmitter excitation voltage on and one with the transmitter off. This methodology may yield image information sets that may not match expected results in terms of fingerprint image definition in a more real-life setting. There might also be the need to tune the operational frequency throughout normal usage, which may lead to inconsistent results.